1. Field of the Invention
The present invention relates to an aqueous solution of a polymer having an upper critical solution temperature, an aqueous dispersion of a particle modified with the polymer and a method of storing the same.
2. Background Art
As a polymer composition showing thermoresponse, a stimulus-responsive magnetic particle obtained by immobilizing polyisopropyl acrylamide, showing a lower critical solution temperature (hereinafter referred to as “LCST”) in a state of an aqueous solution, on a magnetic particle having a particle size of about 100 to 200 nm is known (see, for example, Appl. Microbiol. Biotechnol., vol. 41, pp. 99-105, 1994 (Non-patent document 1), and Journal of Fermentation and Bioengineering, vol. 84, pp. 337-341, 1997 (Non-patent document 2)).
Such a stimulus-responsive magnetic particle is well dispersed in water because it has a particle size of about 100 to 200 nm and is very small. In the case where an aqueous solution of such stimulus-responsive magnetic particles is heated to a temperature not lower than the LCST, the stimulus-responsive magnetic particles aggregate. Since the resulting aggregate can be easily collected by a magnetic force, an attempt has been made to perform separation of various biomolecules or microorganisms using stimulus-responsive magnetic particles on which an antibody or an antigen has been immobilized.
However, in the case of performing separation of a biomolecule or the like using the above-mentioned method, the both temperatures of the target substance and a solution containing the stimulus-responsive magnetic particles are required to be raised to a temperature not lower than the LCST. Therefore, the biomolecules or the like as the target substance may sometimes be damaged or inactivated depending on the LCST of the stimulus-responsive magnetic particles to be used or the thermostability of the target substance.
As another polymer composition showing thermoresponse, a polymer showing an upper critical solution temperature (hereinafter also referred to as “UCST”) in a state of an aqueous solution and a thermoresponsive microparticle obtained by immobilizing this polymer on a carrier particle is known (see, for example, JP-A-2000-086729 (Patent document 1), WO 02/016454 (Patent document 2), and JP-A-2005-082538 (Patent document 3)). In the case where such a thermoresponsive microparticle on which a polymer showing a UCST has been immobilized is used for separation of a biomolecule or the like, the biomolecule or the like as the target substance can be separated and collected without damaging or inactivating the target substance due to heating.
In the case where the above-mentioned thermoresponsive microparticle is used for separation of a biological sample, in order to perform separation of various biomolecules or microorganisms, an antibody or an antigen has been immobilized as a ligand on the thermoresponsive microparticle in many cases. In order to prevent damage or inactivation of such a ligand, the thermoresponsive microparticle is generally stored at a low temperature in the range of from 0° C. to 10° C. However, in many cases, a polymer showing UCST in the range of from 0° C. to 10° C. is used, though depending on the type of thermoresponsive microparticle, therefore, when it is stored at a low temperature, the polymer is to be maintained in an aggregated state. When the polymer is maintained in an aggregated state for a long period of time (for example, for 1 month), even if the temperature is raised to the UCST or higher, the thermoresponsive microparticle may not disperse or it may take time to disperse in some cases.